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Comment by simonhorlick | Hacker News Reader

Anything about 2C is extremely risky for civilised society. It’s likely that we’ve already surpassed the tipping point for the West Antarctic ice sheet which over time will lead to meters of sea level rise. Changing weather patterns and simultaneous breadbasket failures will mean food becomes a lot more expensive - if you’re lucky enough to have access to it at all. At 3.5C many places on the planet become uninhabitable. People aren’t able to work outside for much of the year due to wet bulb temperatures. Regular storm surge causes a large percentage of the planets population to migrate. Salination of ground water and water for crop irrigation becomes a serious problem. Mountain glaciers that provide clean drinking water for millions of people dry up. In all, it’s hard to see a situation where we’d be able to maintain a reasonable quality of life under the conditions of >2C of warming.

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>At 3.5C many places on the planet become uninhabitable

Which places in particular? Here in the middle east temperatures regularly get up to 45-50 degrees celsius in the summer and people get by okay. Most places have average temperatures much lower than that, so how would a rise of just 3-4 degress make then uninhabitable?

Humidity is a big factor in whether heat is bearable or not, and 3.5C is an average over the entire globe. Some places will get much hotter than just 3.5C, some places won't.

There is also humidity. The desert is dry, so you can still perspire and cool down that way; but in a humid place like, say, India it is possible that temperatures will rise high enough that the human body cannot perspire to cool down via evaporation, at which point it would be dangerous to be outside.

Indeed "Wet-bulb temperature" is likely to be the thing that will make several densely populated places in Asia uninhabitable.

Bangladesh alone is +170M people. Dispacing these people will be a refugee crisis on an unseen scale.

+3.5C mean global temperatures means swings of, say, +- 10C in various places at various times.

A 3.5 degree global average increase is much more pronounced on land.

It used to confuse me too..now I think I get it.

The issue isn't all thermometers going up by 3.5 degC. The issue is that it also brings a host of other changes, and it's those things that kill you. They include:

- higher variability of temperatures. So your +3.5 degC actually makes heatwaves of +10/+15 more likely

- changing rainfall patterns, taking rain away from some places that rely on it.

- rising sea level, where the small average temperature changes are sufficient for a significant increase.

I actually think the focus on global average temperature increase is a major failure of public science communication, precisely because everyone mentally adds the increase to their local climate and thinks "meh".

>temperatures regularly get up to 45-50 degrees celsius in the summer and people get by okay

Largely because the 45-50 degree temperature do not last very long. And partly because AC loss due to prolonged blackouts has not happened during such heat - yet.

A 3-4 degree increase in average temperature means that 45-50 degree temperature periods may go from lasting 1-3 days to 2-3 weeks. And that's a whole new ball game.

A quick summary of the takeaways of Vecellio et al.'s recent (October 2023) paper (https://doi.org/10.1073/pnas.2305427120) indicate that at +3.5 °C GMST we should expect high exposure to uncompensable heat stress every year primary around the Indus River Valley (India and surrounding countries) and the highly populated cities in eastern China. In North America, there is some exposure starting at +3°C but until +4 °C those exposures do not become substantial in the typical year.

Another thing noted is that urban features such as green spaces, water features, and increased shade may work where exceedance is typically nonhumid (i.e., North America, Middle East, Australia), these factors may not be as effective or even actively detrimental in more humid environments due to the added humidity.

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